The present invention relates to a method and an apparatus for simultaneously performing hydrothermal reaction and electrolysis. As used herein, hydrothermal electrolysis refers to performing hydrothermal reaction and electrolysis at the same time.
Wastewaters containing organic matters or the like have been treated by hydrothermal reaction. The hydrothermal reaction refers to a reaction which proceeds by holding wastewater or the like at high temperatures under high pressures in the presence of water to decompose organic matters in the wastewater.
During hydrothermal reaction, wastewater its held at an elevated temperature of, for example, 100 to 350xc2x0 C. for, typically, several tens of minutes. For continuous treatment, for example, pressurized wastewater is introduced into a reactor which has been heated so that the wastewater may retain at high temperatures under high pressures for a given period of time. For batch treatment or semi-continuous treatment including repeated batch treatments, wastewater in a pressure-resistant closed container is heated, and pressure is raised with the increasing temperatures.
It is also known to perform the hydrothermal reaction in the presence of an oxidizing agent such as oxygen or hydrogen peroxide for oxidatively decomposing organic matters or the like in wastewater. Such oxidative decomposition reaction is called hydrothermal oxidation reaction or wet oxidation process. In the hydrothermal oxidation reaction or wet oxidation process, compounds having low-molecular weights such as acetic acid or ammonia can not be readily decomposed, and the decomposition rate is limited. Thus, the presence of a heterogeneous catalyst for promoting oxidation reaction was proposed as described in JPB No. 19757/84. The wet oxidation process using a heterogeneous catalyst is also called as catalytic wet oxidation process. In this way, the chemical oxygen demand (COD) of wastewater can be lowered by hydrothermal reaction, which includes wet oxidation process and catalytic wet oxidation process.
Various wastewaters are treated by hydrothermal reaction or wet oxidation process. For example, influent wastewaters include a slurry obtained from organic wastes in a solid, sludge or liquid form such as municipal waste, night soil, sewage sludge and industrial waste. Organic sludge and various industrial wastewaters are also treated. Wastewaters contain various materials, and it is desirable to readily decompose any kind of materials therein.
However, metallic material used in the reactor rapidly corrodes when wastewaters containing salts were treated by the wet oxidation process or catalytic wet oxidation process. When salts were present in the order of percents, they sometimes act as catalyst poison in the catalytic wet oxidation process.
Wastewaters containing salts include, for example, industrial wastewaters, which often contain chloride ions. Contaminated wastewater from power plants sometimes contains 500 to 20,000 mg/L of sodium chloride together with ammonia or monoethanolamine. Leachates from municipal wastes landfill sometimes contain 1,000 to 30,000 mg/L of chloride ions together with COD components. It is not rare that wastewaters from chemical plants and food plants contain tens of thousands of mg/L of chloride ions. Even if these salts are not initially contained, halide ions may be generated in water when wastewaters or wastes containing halogenated organic compounds are treated by hydrothermal reaction.
Thus, special caution was needed to treat wastewaters containing salts such as halide ions. However, the present invention allows halide ions to involve in oxidative decomposition of reduced matters, and the presence thereof improves treatment of wastewaters.
Hydrothermal reaction requires high pressure, and therefore, it has commonly been performed in a closed container. When the internal pressure in the closed container increases above a determined value, a gas releases from an escape valve.
On the other hand, electrolysis in the presence of water generally leads to generation of a hydrogen gas and an an oxygen gas. The presence of the hydrogen gas and the oxygen gas may involve a danger of explosion.
Thus, electrolysis of wastewater in a closed container may mix the hydrogen gas with the oxygen gas to create a danger of explosion. Particularly, when the hydrothermal reaction and electrolysis are performed simultaneously, it would be expected that a mixed gas containing the hydrogen gas and the oxygen gas is brought to high temperatures under high pressures to further increase the force of the explosion.
However, we carefully performed hydrothermal reaction and electrolysis simultaneously in an aqueous solution containing a halide ion on a small scale. To our surprise, we have found that the generation of a hydrogen gas and an oxygen gas is highly suppressed at high temperatures and that reduced matters such as organic matters and ammonia therein are effectively decomposed by oxidation.
According to a first aspect of the present invention, a method for hydrothermal electrolysis is provided comprising the step of applying direct current to an aqueous medium containing water, a halide ion and a reduced matter at a temperature ranging from 100xc2x0 C. to a critical temperature of the aqueous medium under a pressure for maintaining the aqueous medium in the liquid phase.
In the present invention, said aqueous medium preferably is held in a reactor having a metallic inner wall which serves as a cathode while an anode is placed inside of said reactor. When an aqueous medium containing a salt such as halide ions is hydrothermally decomposed, the reactor can be protected from corrosion by cathodic protection.
According to a second aspect of the present invention, an apparatus for hydrothermal electrolysis is provided comprising a reactor capable of withstanding a pressure of a hydrothermal reaction and a pair of electrodes for electrolyzing a matter in the reactor.
According to a third aspect of the present invention, a method for hydrothermal electrolysis is provided comprising the step of applying direct current to an aqueous medium containing water, a strongly acidic ion and a reduced matter at a temperature ranging from 100xc2x0 C. to a critical temperature of the aqueous medium under a pressure for maintaining the aqueous medium in the liquid phase.